Digital communication systems transmit data by various carrier modulation techniques using tracking coherency between a transmit clock and a like receiver clock. Gaussian minimum shift keying (GMSK) is a form of continuous phase modulation having compact spectral occupancy by choosing a suitable bandwidth time product parameter in a Gaussian filter. GMSK is an attractive modulation method for all high throughput frequency division multiple access satellite communication systems where only a limited system bandwidth is available with the transmitters operating at maximum power output efficiency. GMSK communications systems are typically coherent communication systems.
A GMSK receiver for an additive white Gaussian noise channel demodulates the received signal by coherent demodulation into estimated output data stream using a local carrier reference clock. The receiver demodulates by filtering the received signal using a bank of Laurent filters that filter the demodulated received signal into a symbol sequence. A Viterbi decoder searches the symbol sequence for the most probable transmitted data sequence as an estimate of the original NRZ formatted data stream. The symbol sequence has a predetermined alphabet of M symbols as an M-ary symbol modulation method. A typical coherent receiver for a 2-ary GMSK signal is based on a pulse amplitude modulation representation of continuous phase modulated signals using Laurent matched filters matched to the amplitude modulated pulses in the pulse amplitude modulation representation, and further employs the Viterbi algorithm to optimally demodulate the symbol sequence.
U.S. Pat. No. 7,072,414 issued on Jul. 4, 2006 to Lui, entitled GMSK precoding communication method, is directed to a data precoding algorithm implemented in a modulator of a transmitter to substantially improve the resulting bit error rate performance of the continuous phase modulated receivers, such as Gaussian minimum shift keying receivers, without the use of differential decoders while preserving the spectral occupancy of the GMSK signals. The bit error rate is reduced using the precoding method for GMSK signals with memory of L. The GMSK receiver includes a filter bank and Viterbi decoder. A header on the bit stream is used for determining symbol timing for sampling of the output of the filter bank. The timing for sampling is coherent. The header provides a-priori timing for proper sampling of the bit stream. The use of a-priori timing produces inaccurate sampling times. Prior GMSK coherent communications systems have required complicated coherent tracking loops and have increased the sampling rate that requires additional components and power.
Other communication systems use binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), and M-ary phase shift keying. The communication signal is a square wave in nature and digital tracking transition loops are used for noncoherent bit synchronization. That is, the digital tracking transition loop is designed to track the baseband square waves of demodulated received signals without necessarily matching a transmit clock with a like receiving clock. The digital tracking transition loop performs optimally when the square wave signals are received in the presence of the additive white Gaussian noise. Digital transition tracking loops as part of a GMSK timing recover loop have been applied to demodulated GMSK received signals that are highly distorted square wave signals. The GMSK timing recovery loop operates upon the demodulated received GMSK signal using forms of a squaring loop or frequency doubler followed by a phase locked loop for bit timing recovery. The timing clock for timing recovery in the GMSK timing recovery loop is created by squaring the received demodulated signal, and the phase lock loop is tuned to the clock frequency for bit timing recovery. The demodulated GMSK received baseband signal containing data information, however, is severely distorted due to the Gaussian filtering at small bandwidth time products where the 3 dB cut-off frequency of the Gaussian filter is smaller than the data rate of the baseband signaling. Therefore, for small bandwidth time products of GMSK Gaussian filters, both prior GMSK timing recovery loops and digital tracking loops are not capable of recovering the timing information based on the received analog Gaussian filter response waveform.
U.S. Pat. No. 6,411,661, issued to Nguyen on Jun. 25, 2002, entitled digital timing recovery loop for GMSK demodulators, is directed to an improved GMSK timing recover loop in a GMSK receiver for providing a bit synchronization timing signal that is used for reconstructing a data sequence. The GMSK timing recovery loop includes a hard limited and a conventional digital tracking transition loop that has been used in binary phase shift keying (BPSK) and quadrature pulse shift keying (QPSK) for noncoherent communications. The improved GMSK timing recovery loops operates at baseband and provides reduced bit timing synchronization jitter for reducing bit error rates.
GMSK, QPSK, BPSK, and like coherent communications require sampling of the incoming data stream by matching a transmit clock with a like receiver clock and various means have been employed to accurately and rapidly determine the sampling timing based upon coherency. Coherency provides for locking on external transmit clock timing by adjusting delays in an internal clock. Such coherent systems are inherently complicated and required coherent tracking loops. GMSK, QPSK, and BPSK often rely on noncoherent tracking. However, without a dependence upon a precise internal clock, noncoherent tracking is often less effective in recovering an incoming data stream. Noncoherent systems rely on noncoherent tracking of the digital bit stream. However, noncoherent tracking still requires the generation of a precise sampling signal. In a noncoherent digital data communication system an important subsystem function of the receiver is that of recovering the data symbol timing from the received signal that is corrupted by channel noise. Bit time tracking is accomplished using complicated noncoherent tracking loops, or precoding, or headers, while assuming initial bit timing a-priori leading to delays in bit and symbol synchronization and data recovery. These timing functions require high sampling rates that require an excessive amount of power to implement often requiring the excessive use of high speed analog to digital converters. These and other disadvantages are solved or reduced using the invention.